Qitzur Shulchan Arukh – 65:20
If Re’uvein is traveling to a place where they sell some product cheaply, Shim’on is allowed to say to him, “Bring me merchandise from there, and I will give you this-and-this for profit.” As long as the responsibility for the merchandise [i.e. the loss for any loss of merchandise] falls upon Re’uvein until he brings it to Shim’on.
This is another example of defining who owns the merchandise based on who would lose the money if something were to happen to it. If Re’uvein owns the merchandise until the moment it’s delivered to Shim’on, then Shim’on bought it at their agreed upon price. Re’uvein made money conveying the product.
If, however, Shim’on takes ownership when Re’uvein buys it, then he bought it for cheap in the remote location, and is paying Re’uvein extra for the time delay — which is the very definition of prohibited interest.